Cracks and other defects that form within a polymer can be difficult to detect and almost impossible to repair. A successful method of autonomically repairing cracks and other defects that has the potential for significantly increasing the longevity of materials has been described, for example, in U.S. Pat. No. 6,518,330. This self-healing system includes a material containing, for example, solid particles of Grubbs catalyst and capsules containing liquid dicyclopentadiene (DCPD) embedded in an epoxy matrix. When a crack propagates through the material, it ruptures the microcapsules and releases DCPD into the crack plane. The DCPD then mixes with the Grubbs catalyst, undergoes Ring Opening Metathesis Polymerization (ROMP), and cures to provide structural continuity where the crack had been.
Crack formation in coatings can be especially problematic, since coatings are often present to protect the substrate onto which they have been coated. For example, metal substrates may be coated with a layer of material to prevent or inhibit corrosion of the metal. A crack in such a coating typically leads to corrosion of the underlying metal, resulting in expensive and wasteful repair or replacement of some or all of a part made from the metal. To ensure the integrity of the metal, it may be necessary to replace the coating periodically, regardless of whether cracks actually have formed. Coatings also may be used in the form of primers, paints, stains, sealers and topcoats. Substrates for these coatings include building materials, windows, electronics, automotive parts, marine parts and aerospace parts. These coatings may protect the underlying material from corrosion, moisture, bacterial growth, ultraviolet radiation and/or mechanical impact.
Extrinsic self-healing is a straightforward approach to healing damaged materials and elongating material lifetime. This is a type of process that is initiated by healing agents released from micro- or nano-scale containers that were embedded in a substrate. However, this process can require long healing times and high healing temperatures due to limited loading of healing agents, slow delivery of healing agents to the damage sites, and slow reaction kinetics. At an early stage of a damage event, such as when cracks are microscopic in size, or even before observable damage occurs, self-strengthening, a specific subgroup of self-healing, can be used as a preventive strategy to increase material strength and prevent further material damage. A significant portion of self-strengthening materials are stimuli responsive and able to convert otherwise neutral or destructive environmental triggers (e.g., mechanical force, light, acids, bases, etc.) to constructive processes.
It is desirable to provide a self-strengthening, and even more broadly self-healing systems in which minor triggers (such as bond scission, the rupture of only a few microcapsules, or the activation of a mechanophore) can result in strengthening of a material at a lower temperature and at a faster rate, and so materials which can take a minor event, and turn it into a significant response are highly desirable.